This invention relates generally to hydrocarbon processing and, more particularly, to the processing of hydrocarbon-containing materials having a high light olefin content, such as produced or formed in or by the cracking of a heavy hydrocarbon feedstock.
Light olefins serve as feed materials for the production of numerous chemicals. Light olefins have traditionally been produced through the processes of steam or catalytic cracking of hydrocarbons such as derived from petroleum sources. Fluidized catalytic cracking (FCC) of heavy hydrocarbon streams is commonly carried out by contacting a starting material whether it be vacuum gas oil, reduced crude, or another source of relatively high boiling hydrocarbons with a catalyst such as composed of finely divided or particulate solid material. The catalyst is transported in a fluid-like manner by transmitting a gas or vapor through the catalyst at sufficient velocity to produce a desired regime of fluid transport. Contact of the oil with the fluidized material catalyzes the cracking reaction.
The cracking reaction typically deposits coke on the catalyst. Catalyst exiting the reaction zone is commonly referred to as being “spent”, i.e., partially deactivated by the deposition of coke upon the catalyst. Coke is comprised of hydrogen and carbon and can include, in trace quantities, other materials such as sulfur and metals such that may enter the process with the starting material. The presence of coke interferes with the catalytic activity of the spent catalyst. It is believed that the coke blocks acid sites on the catalyst surface where the cracking reactions take place. Spent catalyst is traditionally transferred to a stripper that removes adsorbed hydrocarbons and gases from catalyst and then to a regenerator for the purpose of removing the coke by oxidation with an oxygen-containing gas. An inventory of catalyst having a reduced coke content, relative to the spent catalyst in the stripper, hereinafter referred to as regenerated catalyst, is collected for return to the reaction zone. Oxidizing the coke from the catalyst surface releases a large amount of heat, a portion of which escapes the regenerator with gaseous products of coke oxidation generally referred to as flue gas. The balance of the heat leaves the regenerator with the regenerated catalyst. The fluidized catalyst is continuously circulated between the reaction zone and the regeneration zone. The fluidized catalyst, as well as providing a catalytic function, serves as a vehicle for the transfer of heat from zone to zone. FCC processing is more fully described in U.S. Pat. No. 5,360,533 to Tagamolila et al., U.S. Pat. No. 5,584,985 to Lomas, U.S. Pat. No. 5,858,206 to Castillo and U.S. Pat. No. 6,843,906 B1 to Eng, the contents of each of these patents are hereby incorporated herein by reference. Specific details of the various contact zones, regeneration zones, and stripping zones along with arrangements for conveying the catalyst between the various zones are well known to those skilled in the art.
The FCC reactor serves to crack gas oil or heavier feeds into a broad range of products. Cracked vapors from an FCC unit enter a separation zone, typically in the form of a main column, that provides a gas stream, a gasoline cut, light cycle oil (LCO) and clarified oil (CO) which includes heavy cycle oil (HCO) components. The gas stream may include dry gas, i.e., hydrogen and C1 and C2 hydrocarbons, and liquefied petroleum gas (“LPG”), i.e., C3 and C4 hydrocarbons, also sometimes commonly referred to as wet gas.
As a result or through such hydrocarbon cracking processing, byproduct species such as CO2, H2S and other sulfur compounds may form or otherwise be present in the FCC effluent in undesirably high relative amounts. In the past, amine units have been used to separate species such as CO2 from hydrocarbon stream materials. In typical amine systems, an amine solvent such as methyl diethanol amine [MDEA] is used to absorb or otherwise separate CO2 from hydrocarbon stream materials. A stripper is typically subsequently used to strip the absorbed CO2 from the amine solvent, permitting the reuse of the stripped amine solvent.
In view of an increasing need and demand for light olefins such as ethylene and propylene for various petrochemical uses such as for the production of polyethylene, polypropylene and the like as well as the desire to produce relatively less of heavier olefins such as butylenes and pentenes which are generally less desirable as gasoline blending components due to environmental considerations, it may be desired to practice the cracking reaction processing of heavy hydrocarbon feedstock to increase the relative amount of light olefins in the resulting product slate.
Research efforts have led to the development of an FCC process that produces or results in greater relative yields of light olefins, i.e., ethylene and propylene. Such processing is more fully described in U.S. Pat. No. 6,538,169 B1 to Pittman et al., the contents of which are hereby fully incorporated herein by reference. As disclosed therein, a hydrocarbon feed stream can desirably be contacted with a blended catalyst comprising regenerated catalyst and coked catalyst. The catalyst has a composition including a first component and a second component. The second component comprises a zeolite with no greater than medium pore size wherein the zeolite comprises at least 1 wt. % of the catalyst composition. The contacting occurs in a riser to crack hydrocarbons in the feed stream and obtain a cracked stream containing hydrocarbon products including light olefins and coked catalyst. The cracked stream is passed out of an end of the riser such that the hydrocarbon feed stream is in contact with the blended catalyst in the riser for less than or equal to 2 seconds on average.
As with conventional FCC processing, byproduct species such as CO2, H2S and other sulfur compounds may form or otherwise be present in the resulting effluent in undesirably high relative amounts. However, whereas conventional FCC processing effluent streams typically have little if any olefin content, the such modified hydrocarbon processing desirably produces or results in an effluent stream having a significantly large olefin content. With standard amine system treatment of such large olefin content effluent streams, some of the olefin material is typically co-absorbed with the CO2 in or by the amine solvent. Such co-absorption of olefin material undesirably reduces the amounts of light olefins available for recovery from such processing.
Moreover, in conventional amine treatment processing, the amine solvent containing the absorbed CO2 is typically subjected to further processing such as through a stripper wherein the absorbed CO2 can desirably be separated from the amine solvent and the amine solvent can be recycled and reused for amine treatment of a selected carbon dioxide-containing stream. Unfortunately, during such subsequent stripper processing of the amine solvent, the presence of such olefin materials can lead to polymerization. Such polymerization can lead to degradation of the amine solvent and require expensive off-site reclamation processing.
Thus, there is a need and a demand for processing and arrangements for increased effectiveness for the separation and removal of carbon dioxide from high olefin content process streams.
More particularly, there is a need and a demand for improved amine treatment processing arrangements and processing schemes of such high olefin content process streams such as for the effective separation and removal of carbon dioxide therefrom while desirably permitting increased or improved olefin recovery.